In magneto-optical storage systems that use magneto-optical (MO) recording material deposited on a rotating disk, information may be recorded on the disk as spatial variations of magnetic domains. During readout, a magnetic domain pattern modulates an optical polarization, and a detection system converts a resulting signal from optical to electronic format.
In one type of a magneto-optical storage system, a magneto-optical head assembly is located on an actuator that moves the head to position the head assembly over data tracks during recording and readout. A magnetic coil is used to create a magnetic field that has a magnetic component in a direction perpendicular to the disk surface. A vertical magnetization of polarity, opposite to that of the surrounding magnetic material of the disk medium, is recorded as a mark indicating zero or a one by first focusing a beam of laser light to form an optical spot on the disk. The optical spot functions to heat the magneto-optical material to a temperature near or above a Curie point (a temperature at which the magnetization may be readily altered with an applied magnetic field). A current passed through the magnetic coil orients the spontaneous vertical magnetization either up or down. This orientation process occurs in the region of the optical spot where the temperature is suitably high. The orientation of the magnetization mark is preserved after the laser beam is removed. The mark is erased or overwritten if it is locally reheated to the Curie point by the laser beam during a time the magnetic coil creates a magnetic field in the opposite direction.
Information is read back from a particular mark of interest on the disk by taking advantage of the magnetic Kerr effect so as to detect a Kerr rotation of the optical polarization that is imposed on a reflected beam by the magnetization at the mark of interest. The magnitude of the Kerr rotation is determined by the material's properties (embodied in the Kerr coefficient). The sense of the rotation is measured by established differential detection schemes and, depending on the direction of the spontaneous magnetization at the mark of interest, is oriented clockwise or counter-clockwise.
Conventional magneto-optical heads tend to be based on relatively large optical assemblies which make the physical size and mass of the head rather bulky (typically 3-15 mm in a dimension). Consequently, the speed at which prior art magneto-optical heads are mechanically moved to access new data tracks on a magneto-optical storage disk is slow. Additionally, the physical size of the prior art magneto-optical heads limits the spacing between magneto-optical disks. Because the volume available in standard height disk drives is limited, magneto-optical disk drives have not been available as high capacity commercial products.
N. Yamada (U.S. Pat. No. 5,255,260) discloses a flying optical head for accessing an upper and lower surface of a plurality of optical disks. The flying optical head disclosed by Yamada describes an actuating arm that has a static (fixed relative to the arm) mirror or prism mounted thereon, for delivering light to and receiving light from a phase-change optical disk. While the static optics described by Yamada provides access to both surfaces of a plurality of phase-change optical disks contained within a fixed volume, Yamada is limited by the size and mass of the optics. Consequently, the performance and the number of optical disks that can be manufactured to function within a given volume is also limited.
Utilization of optical fibers to deliver light to a storage location within an optical disk drive allows for a lower profile optical path which can increase the number of disks that can be vertically positioned within a given form factor. In a magneto-optical storage drive, polarization-maintaining optical (PM) fiber is typically used to convey laser light delivered by inexpensive laser diodes. The mode field that is exhibited by single-mode optical fiber is typically circularly symmetric. Because the mode field of a Fabry Perot laser diode is typically asymmetric, the mode field mismatch may create optical inefficiencies that result in a reduced signal to noise ratio in the detected data signal.
What is needed, therefore, is a method and apparatus that enables a laser source exhibiting an asymmetric mode field to be used with polarization maintaining optical fiber so as to efficiently convey light between a laser source and a storage location of an optical data storage system.